Fasted training: a case for exercising on an empty stomach

It could help your body become better at using the right fuel for the exercise you’re doing. We took a closer look

When it comes to working out, is food necessarily always the best fuel? Whether it’s a pre-workout smoothie or a carb-packed breakfast to get the body into gym mode, eating beforehand has become commonplace in many people’s fitness regimes. However, fasted training, the latest facet to the fasting phenomenon, seeks to turn the concept on its head.

“Fasted exercise means exercising in the fasted state - once your glycogen (stored carbohydrate) stores have been depleted (about 12 hours after your last meal),” explains personal trainer and fasted exercise advocate, Max Lowery. The theory goes that by reducing the body’s dependence on carbohydrates for energy, the way’s cleared for the body to tap into its fat reserves for certain activities instead - a state otherwise known as becoming ‘fat adapted.’

Coming across the subject four years ago, the way fasted training made Max feel rather than the more complex mechanisms behind it is what initially attracted him to it. “I found that having a full stomach slowed me down and I felt heavy and lethargic,” he says. “I initially started doing long fasted hikes, I then started to incorporate it into my strength training. I never wanted to depend on anything to perform at my best - I always wanted to see what my body could do and teach it to be as self-sufficient as possible.” After he returned, he was inspired to research into it more and continued doing it while competing nationally as a sprinter. During this time, he followed what he calls the “train-hard, race-easy” principle. This involved training in a fasted state in order to help his body become better at maximising energy reserves and then racing with high carbohydrate stores by eating beforehand on race days to maximise his performance.

How does it work?

It is thought that by depriving the body of food before your workouts (some, not all), it is then less likely to rely on carbohydrates for energy and will therefore burn existing fat stores to fuel it instead. The period of time in which this happens is called the ‘fasted state.’ Max explains that glucagon is released to keep blood sugar levels normal during this time, after which the body begins to breaks down fat tissue into free fatty acids. These can be converted into a form of energy known as ketone bodies. It’s the tapping into of this fat-burning mechanism that lies at the concept’s core. If we’re constantly feeding the body, the theory is that we never give it an opportunity to ‘switch it on.’

It’s a common misconception though that the main objective of fasted training is to increase weight loss, warns Max. “There is a lot of confusion about the benefits behind fasted training,” he says. “A lot of people think that it is a magic pill for weight loss, unfortunately there are no studies to back up this claim – it may have a weight loss effect for some but nothing has been categorically proven, more research needs to be done.”

“The main benefit that some studies have confirmed is that it makes you more metabolically efficient. This means your body becomes better at using the right fuel source for the exercise or activity intensity,” he explains. “Fat is the preferred fuel source for low intensity aerobic activities like our day to day activities, walking, jogging, cycling and swimming. Once the intensity goes up, carbohydrates have been shown to have an effect on performance for activities that involve speed, power and explosiveness. Being more metabolically efficient will mean that you won’t crave high energy foods throughout the day because your body will be perfectly capable at fuelling itself – using stored body fat.”

Our carb-dependant lifestyles have made it increasingly more difficult for our bodies to use the most beneficial fuel for the right activity. “This [using stored body fat] is in fact our natural way of being, but when we graze on food all day you become dependent on the sugars in the food for energy," says Max. "The mid-morning and mid-afternoon slumps in energy levels are a symptom of this dependence.”

It is thought that by incorporating fasted training into some of your workouts, your body becomes better equipped to use your resources in the best way. “Getting your body used to using both energy stores is important - if you only ever train in the ‘fed’ state, it will mean it is much more difficult to tap into fat reserves (unless you are following an intermittent fasting protocol),” he says.

From a storage perspective, it seems to make sense too. “Fat is an abundant fuel source (even a lean person of bodyweight 70kg, stores around 90,000-140,000 kcal of fat – sufficient to allow a person to walk 16,000km),” Max explains. “While carbohydrate stores are limited (1,800 kcal to 2,500 kcal in a fed state). Therefore, increasing the reliance on fat as a fuel during exercise could be viewed as a favourable adaptation as it will help preserve limited carbohydrate stores and possibly extend time to fatigue.”

Who’s it for?

If like Max, eating before a workout often leaves you feeling lethargic and sluggish during it, fasted training could be worth trying. “It should be used as a tool here and there to maximise performance, particularly in marathon training and other endurance events,” says Max. “As I mentioned above, it can be a very useful tool for optimising your body’s ability to use the right fuel source. It also boils down to personal preference, if you feel better training on an empty stomach – whatever the intensity, then keep it up! You are the best judge of you.”

Are both men and women likely to see the same results? It depends. From a weight loss perspective, the findings from this experiment conducted on Trust Me I’m a Doctor, showed that men burned less fat if they’d eaten carbs before exercise, whereas women burned more fat if they had. However, as Max has highlighted, this way of training is more about encouraging the body to use the right energy sources for the right types of exercises and less about fat loss. When it comes to his clients, he approaches it on a case by case basis. “Some of my female clients love fasted training, others don’t,” he says. “It comes down to personal preference and what kind of training you are doing. Fasted exercise is not a secret pill for weight loss. My clients understand this, so if they do decide to try it, it’s for the right reasons. The Trust Me I’m a Doctor experiment was interesting, but by no means is it conclusive. I am generally wary of 'POP Science' BBC experiments as they can lead to thousands of people to think that that is the final answer, it works both ways.”

A woman who’s seen the benefits of fasting training first-hand is swimwear designer, Alexandra Miro, who first came across it through the free 6 day email course on intermittent fasting her husband’s company, Form Nutrition, runs. Written by their Head of Nutrition, Dr Adam Collins, who is the Director of Nutrition at the University of Surrey and conducts research on fasting, it served as a useful resource and she’s since incorporated it into her weekly fitness regime. “Every day I fast for 18 hours,” she tells us. “It sounds worse than it is - effectively I have dinner and then don't eat until the following lunchtime. I will do light cardio or weight training fasted in the morning before lunch. This actually works really well for me and not having breakfast is a real time saver!”

What are the specific benefits and negatives that she’s experienced? “I feel leaner and more focused in the mornings for sure,” she says. “Fasting in the mornings also saves me time when I'm getting ready for work or taking my daughter to school. I've not seen any negatives but understand I shouldn't do HIIT fasted, so I save that for the evenings.”

Who’s it not for?

If you’ve had previous difficulties with regards to your relationships with food or exercise, fasted training isn’t the healthiest option. “It is not for people with a history of eating disorders, pregnant women, anyone with adrenal fatigue or anyone who has a negative relationship towards their training and exercise,” cautions Max. “If your only motivation to exercising is weight loss, then fasted training isn’t for you. Having a negative body image as the focus for your training will never lead to lasting results, your motivation will be temporary and you will never enjoy the process. I see many people over train all the time because they think it will lead to weight loss and adding fasted training on top of this will create too much stress for the body which can have a very negative impact.”

For those who aren’t used to exercising on an empty stomach, expect to feel a few physical side-effects to begin with. “There may be feelings of lethargy and light headedness but this is just your body adjusting to switching fuel sources, it should pass after a week,” says Max. If you feel better exercising after eating though, go with your gut. It’s not for everyone and your intuition plays a pivotal role.

What's the best way to try it?

1. Try intermittent fasting

“The easiest way to start fasted exercise is to start any method of intermittent fasting, like The 2 Meal Day,” says Max. “This will teach your body to become fat adapted which means you will be using fat as fuel for the right exercise intensities. Start by pushing your first meal a bit later into the day, see how you handle it and then experiment with fasted exercise once per week for something fairly low intensity. Listen to your body and don’t overdo it. Once you get used to it, you can incorporate the ‘train low, race easy’ principle – do some of your training fasted, allowing your body to become as efficient as possible at supplying itself with energy, and then if you have a race or want to perform at your best, you would consume food prior to this. I used this tactic as a competitive sprinter.”

2. Mix it up

“It is important to mix it up and don’t do all your workouts fasted,” Max writes on his website. “If you know the workout is going to be more intense and cardiovascular based, I suggest consuming carbohydrates beforehand to help with performance and to prevent low blood sugar which can cause dizziness and nausea.”

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